A junction field-effect transistor, in which a compound semiconductor layer is used for an electron transport layer and the like, is useful as a high-power semiconductor device. Surfaces of the compound semiconductor layers used for the semiconductor devices are chemically and physically weak, when compared with surfaces of silicon layers or the like. Accordingly, once oxidized, the surface of a compound semiconductor layer allows the generation of electron traps or the like and thus is not suitable any more for practical use.
For this reason, when this type of semiconductor device is manufactured, a protective insulating film is formed on the surface of the uppermost compound semiconductor layer so as to prevent the compound semiconductor layer from being oxidized during manufacturing processes.
Various examples of the protective insulating film are disclosed in the following patent Documents 1 to 4 (Japanese Unexamined Patent Application Laid-Open Publication No. Hei 4-6835, Japanese Unexamined Patent Application Laid-Open Publication No. Hei 3-240265, Japanese Unexamined Patent Application Laid-Open Publication No. 2000-323495, Japanese Patent No. 2792948).
For example, in the patent Document 1 (Japanese Unexamined Patent Application Laid-Open Publication No. Hei 4-6835), a film in which a first silicon nitride (SiN) film with high hydrogen content and a second silicon nitride film with low hydrogen content are stacked in this order is used as a protective insulating film.
However, this protective insulating film has a disadvantage that hydrogen moves from the first silicon nitride film with high hydrogen content to a substrate and then neutralizes donors, thereby making it difficult for a current to flow between a source electrode and a drain electrode.
As described above, in a semiconductor device including a compound semiconductor layer, it is important to stabilize a surface state of the compound semiconductor layer and to improve the electric characteristics thereof, by optimizing the film quality of a silicon nitride film functioning as a protective insulating film.